U.S. patent application number 10/784079 was filed with the patent office on 2004-09-16 for induction heating or melting power supply utilizing a tuning capacitor.
Invention is credited to Fishman, Oleg S., Georgopoulos, George.
Application Number | 20040178680 10/784079 |
Document ID | / |
Family ID | 34911416 |
Filed Date | 2004-09-16 |
United States Patent
Application |
20040178680 |
Kind Code |
A1 |
Fishman, Oleg S. ; et
al. |
September 16, 2004 |
Induction heating or melting power supply utilizing a tuning
capacitor
Abstract
A rectifier/inverter power supply for use with induction heating
or melting apparatus includes a tuning capacitor connected across
the output of the rectifier and input of the inverter. The tuning
capacitor forms a resonant circuit with an inductive load coil at
the operating frequency of the inverter. Additionally, the load
coil may be formed from an active load coil connected to the output
of the inverter and a passive load coil, in parallel with a
resonant tuning capacitor.
Inventors: |
Fishman, Oleg S.; (Maple
Glen, PA) ; Georgopoulos, George; (Pine Brook,
NJ) |
Correspondence
Address: |
PHILIP O. POST
INDEL, INC.
PO BOX 157
RANCOCAS
NJ
08073
US
|
Family ID: |
34911416 |
Appl. No.: |
10/784079 |
Filed: |
February 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10784079 |
Feb 21, 2004 |
|
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10217081 |
Aug 12, 2002 |
|
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6696770 |
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60312159 |
Aug 14, 2001 |
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Current U.S.
Class: |
307/17 |
Current CPC
Class: |
H02M 5/458 20130101;
Y02B 70/10 20130101; H05B 6/04 20130101; H02M 7/4815 20210501; H05B
6/067 20130101 |
Class at
Publication: |
307/017 |
International
Class: |
H02J 003/00 |
Claims
1. A power supply for inductively heating or melting an
electrically conductive material, the power supply comprising: a
rectifier for converting an ac input power into a dc output power
at the output of the rectifier; an inverter having an input
connected to the output of the rectifier, the inverter converting
the dc output power of the rectifier into an ac output current
supplied to an output of the power supply, the ac output current
having a frequency equal to the operating frequency of the
inverter; an at least one tuning capacitor connected across the
output of the rectifier and the input of the inverter; an
autotransformer connected to the output of the power supply, the
autotransformer having a first autotransformer output terminal and
a plurality of second autotransformer output terminals, the
plurality of second autotransformer output terminals comprising at
least two autotransformer taps; and an at least one inductive load
coil connected across the first autotransformer terminal and one of
the plurality of second autotransformer terminals, the at least one
inductive load coil, in combination with the connected impedance of
the autotransformer, having an impedance so that it is at least
approximately in resonance with the at least one tuning capacitor
at the operating frequency of the inverter, whereby the
electrically conductive material is inductively heated or melted by
a magnetic field generated from the flow of the ac output current
through the at least one inductive load coil.
2. A method of inductively heating or melting an electrically
conductive material, the method comprising the steps of: rectifying
an ac input power into a dc output power; inverting the dc output
power to produce an output ac current from an inverter at an
operating frequency of the inverter; connecting the output ac
current to an autotransformer, the autotransformer having a first
autotransformer terminal and a plurality of second autotransformer
terminals comprising at least two autotransformer taps; connecting
an at least one inductive load coil across the first
autotransformer terminal and one of the plurality of second
autotransformer terminals to generate a magnetic field that
magnetically couples with the electrically conductive material to
inductively heat or melt the electrically conductive material; and
forming an at least approximately resonant circuit at the operating
frequency of the inverter with the at least one inductive load coil
in combination with the connected impedance of the autotransformer,
and an at least one tuning capacitor disposed across the dc output
power.
3. A power supply for inductively heating or melting an
electrically conductive material, the power supply comprising: a
rectifier for converting an ac input power into a dc output power
at the output of the rectifier, the output of the rectifier
comprising a positive dc bus and a negative dc bus; an inverter
having a dc input connected to the output of the rectifier, the
inverter comprising a first pair of first and third switch/diode
assemblies and a second pair of second and fourth switch/diode
assemblies, the four switch/diode assemblies forming a full bridge
inverter with the first and second switch/diode assemblies each
having a first terminal, in combination the two first terminals
forming a positive dc inverter input, the positive dc inverter
input connected to the positive dc bus, and the third and fourth
switch/diode assemblies each having a first terminal, in
combination the two first terminals forming a negative dc inverter
input, the negative dc inverter input connected to the negative dc
bus, the first and fourth switch/diode assemblies having a second
terminal commonly connected together to form a first ac inverter
output connection, the second and third switch/diode assemblies
having a second terminal commonly connected together to form a
second ac inverter output connection, the inverter converting the
dc output power of the rectifier into an ac output current supplied
to an output of the power supply, the ac output current having a
frequency equal to the operating frequency of the inverter; an at
least one tuning capacitor having a first and second tuning
capacitor terminals, the first and second tuning capacitor
terminals connected across the positive dc inverter input and the
negative dc inverter input, respectively, the connection between
the first tuning capacitor terminal and the positive dc inverter
input formed from a thin electrically conductive sheet, the
connection between the second tuning capacitor terminal and the
negative dc inverter input formed from a second thin electrically
conductive sheet, the first and second electrically conductive
sheets separated by a thin layer of high dielectric electrical
insulation and joined together to form a low inductance connection;
an at least one inductive load coil connected across the first and
second ac inverter output connections, the at least one inductive
load coil having an inductance so that it is at least approximately
in resonance with the at least one tuning capacitor at the
operating frequency of the inverter, whereby the electrically
conductive material is inductively heated or melted by a magnetic
field generated from the flow of the ac output current through the
at least one inductive load coil.
4. The power supply of claim 3 wherein the at least one inductive
load coil further comprises an active inductive load coil and an at
least one passive inductive load coil, the at least one passive
inductive load coil not connected to the active inductive load
coil, the at least one passive inductive load coil connected in
parallel with an at least one resonant passive circuit tuning
capacitor to form a parallel tank resonant circuit, the passive
inductive load coil magnetically coupled with the active inductive
load coil when the ac output current flows through the active
inductive load coil to induce a secondary ac current in the
parallel tank resonant circuit, the impedance of the combination of
the active inductive load coil and the parallel tank resonant
circuit at least approximately in resonance with the impedance of
the at least one tuning capacitor at the operating frequency of the
inverter.
5. The power supply of claim 3 wherein the at least one tuning
capacitor comprises a plurality of wound film capacitors, each of
the plurality of wound film capacitors having a first and second
capacitor conductors, all of the first capacitor conductors
connected to a first electrically conductive capacitor sheet, and
all of the second capacitor conductors connected to a second
electrically conductive capacitor sheet, the first and second
electrically conductive capacitor sheets separated by a thin layer
of high dielectric electrical insulation and joined together to
form a low inductance connection, the first electrically conductive
capacitor sheet forming the first tuning capacitor terminal, and
the second electrically conductive capacitor sheet forming the
second tuning capacitor terminal.
6. The power supply of claim 5 wherein the plurality of wound film
capacitors comprises a first group of wound film capacitors, each
of the first group of wound film capacitors having their first
capacitor conductors in contact with the first electrically
conductive capacitor sheet, and a second group of wound film
capacitors, each of the second group of wound film capacitors
having their first capacitor conductors in contact with the second
electrically conductive capacitor sheet, each of the first group of
wound film capacitors having their second conductors in contact
with the second electrically conductive capacitor sheet, and each
of the second group of wound film capacitors having their first
capacitor conductors in contact with the second electrically
conductive capacitor sheet.
7. The power supply of claim 6 wherein at least either the first or
second electrically conductive capacitor sheet is pressed at least
partially over each of the plurality of wound film capacitors.
8. A method of inductively heating or melting an electrically
conductive material, the method comprising the steps of: rectifying
an ac output power into a dc output power at the output of a
rectifier, the output of the rectifier comprising a positive dc bus
and a negative dc bus; forming an inverter from a first pair of
first and third switch/diode assemblies and a second pair of second
and fourth switch/diode assemblies, the four switch/diode
assemblies forming a full bridge inverter with the first and second
switch/diode assemblies each having a first terminal, in
combination the two first terminals forming a positive dc inverter
input, the positive dc inverter input connected to the positive dc
bus, and the third and fourth switch/diode assemblies each having a
negative dc inverter input, the negative dc inverter input
connected to the negative dc bus, the first and fourth switch/diode
assemblies having a second terminal commonly connected together to
form a first ac output inverter connection, the second and third
switch/diode assemblies having a second terminal commonly connected
together to form a second ac output inverter connection, the
inverter converting the dc output power of the rectifier into an ac
output current supplied to an output of the power supply, the ac
output current having a frequency equal to the operating frequency
of the inverter; connecting an at least one tuning capacitor having
a first and second tuning capacitor terminals across the positive
and negative dc inverter inputs, the first connection between the
first tuning capacitor terminal and the positive dc inverter input
formed from a first thin electrically conductive sheet, the second
connection between the second tuning capacitor terminal and the
negative dc inverter input formed from a second thin electrically
conductive sheet; separating the first and second thin electrically
conductive sheets separated by a thin layer of high dielectric
electrical insulation; joining the first and second thin
electrically conductive sheets together with the intervening thin
layer of high dielectric electrical insulation to form a low
inductance connection; connecting the first and second ac inverter
outputs to an at least one inductive load coil to generate a
magnetic field that magnetically couples with the electrically
conductive material to inductively heat or melt the electrically
conductive material; and forming an at least approximately resonant
circuit at the operating frequency of the inverter with the at
least one inductive load coil and an the least one tuning
capacitor.
9. The method of claim 8 further comprising the steps of:
inductively coupling a passive inductive load coil to the magnetic
field generated by the at least one inductive load coil, the
passive inductive load coil connected in parallel with an at least
one resonant passive circuit tuning capacitor to form a parallel
tank resonant circuit; and forming an at least approximately
resonant circuit at the operating frequency of the inverter with
the impedance of the combination of the at least one inductive load
coil and the parallel tank resonant circuit, and the at least one
tuning capacitor.
10. The method of claim 8 further comprising the steps of: forming
the at least one tuning capacitor from a plurality of wound film
capacitors; connecting a first wound film capacitor terminal of
each of the plurality of wound film capacitors to a first tuning
capacitor connecting conductor, the first tuning capacitor
connecting conductor formed from a third thin electrically
conductive sheet; connecting a second wound film capacitor terminal
of each of the plurality of wound film capacitors to a second
tuning capacitor connecting conductor, the second tuning capacitor
connecting conductor formed from a fourth thin electrically
conductive sheet; separating the third and forth thin electrically
conductive sheets separated by a thin layer of high dielectric
electrical insulation; joining the third and fourth thin
electrically conductive sheets together with the intervening thin
layer of high dielectric electrical insulation to form a low
inductance connection; forming the first tuning capacitor terminal
from the third thin electrically conductive sheet; and forming the
second tuning capacitor terminal from the fourth thin electrically
conductive sheet.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/217,081, filed Aug. 12, 2002, which claims
priority to provisional patent application serial No. 60/312,159,
filed Aug. 14, 2001, the entirety of each of which are incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an ac power supply for use
in induction heating or melting applications wherein the induction
power circuit is resonantly tuned.
BACKGROUND OF THE INVENTION
[0003] FIG. 1 illustrates a conventional power supply 110 that is
used in induction heating or melting applications. The power supply
consists of an ac-to-dc rectifier and filter section 112, a
dc-to-ac inverter section 120 and a tuning capacitor section 130.
For the power supply shown in FIG. 1, a three-phase diode bridge
rectifier 114 converts three-phase (A, B, C) ac utility line power
into dc power. Current limiting reactor L.sub.108 smoothes out the
ripple in the output dc current of the rectifier, and capacitor
C.sub.108 filters the ac component from the output dc voltage of
the rectifier. The filtered dc output of the rectifier is inverted
to ac by a full-bridge inverter consisting of solid state switches
S.sub.101, S.sub.102, S.sub.103 and S.sub.104 and associated
antiparallel diodes D.sub.101, D.sub.102, D.sub.103 and D.sub.104,
respectively. Alternating turn-on/turn-off cycles of switch pairs
S.sub.101/S.sub.103 and S.sub.102/S.sub.104 produce a synthesized
ac inverter output at terminals 3 and 4.
[0004] Induction load coil L.sub.101, represents the power coil
used in the induction heating or melting application. For example,
in an induction furnace, load coil L.sub.101, is wound around the
exterior of a crucible in which metal charge has been placed. In an
induction heating application, a metal workpiece, such as a strip
or wire, may travel through a helical winding of load coil
L.sub.101, or otherwise be brought near to the coil to inductively
heat the workpiece. Current supplied by the power supply and
flowing through load coil L.sub.101 creates a magnetic field that
either directly heats the metal charge or workpiece by magnetic
induction, or heats the workpiece by heat conduction from a
susceptor that is heated by magnetic induction. Load coil
L.sub.101, whether it be a single coil or an assembly of
interconnected coil sections, has a very low operating power
factor. Because of this, a tuning capacitor (or bank of
capacitors), such as capacitor C.sub.101 must be provided in the
load coil circuit to improve the overall power factor of the load
coil circuit. These tuning capacitors are a significant cost and
volume component of the power supply. Therefore, there exists the
need for a power supply for inductive heating or melting
applications that utilizes smaller and less costly tuning
capacitors.
[0005] An objective of the present invention is to provide a power
supply for inductive heating or melting applications that utilizes
a capacitor connected between the output of the rectifier and the
input of the inverter to form a resonantly tuned circuit with the
induction load coil used in the application.
BRIEF SUMMARY OF THE INVENTION
[0006] In one aspect, the present invention is apparatus for, and a
method of, providing a power supply with rectifier and inverter
sections for use with an induction load coil wherein a tuning
capacitor is provided across the output of the rectifier and the
input of the inverter to form a resonant circuit with the induction
load coil. The induction load coil may comprise an active load coil
connected to the output of the inverter, and a passive load coil
connected in parallel with a capacitor to form a tank circuit.
Other aspects of the invention are set forth in this specification
and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] For the purpose of illustrating the invention, there is
shown in the drawings a form that is presently preferred; it being
understood, however, that this invention is not limited to the
precise arrangements and instrumentalities shown.
[0008] FIG. 1 is a schematic diagram of a prior art power supply
with a full-bridge inverter that is used in induction heating and
melting applications.
[0009] FIG. 2 is a schematic diagram of one example of the power
supply of the present invention for use in induction heating or
melting applications.
[0010] FIG. 3 is a waveform diagram illustrating the inverter's
output voltage and current for one example of the power supply of
the present invention.
[0011] FIG. 4 is a waveform diagram illustrating the voltage across
a tuning capacitor and the current through a line filtering reactor
used in one example of the power supply of the present
invention.
[0012] FIG. 5 is a waveform diagram illustrating the voltage
across, and current through, a switching device used in the
inverter in one example of the power supply of the present
invention.
[0013] FIG. 6 is a schematic diagram of another example of the
power supply of the present invention for use in induction heating
or melting applications.
[0014] FIG. 7 is a vector diagram illustrating the advantages of an
induction heating or melting system with the power supply of the
present invention used with the load coil system illustrated in
FIG. 6.
[0015] FIG. 8 is a schematic diagram of another example of the
power supply of the present invention for use in induction heating
or melting applications.
[0016] FIG. 9 is an isometric of one example of the physical
arrangement of the inverter and tuning capacitor used in the power
supply of the present invention.
[0017] FIG. 10 is a top view of one example of the physical
arrangement of the inverter used in the power supply of the present
invention.
[0018] FIG. 11(a) is a cross sectional view of the physical
arrangement of the inverter shown in FIG. 10 along line A-A.
[0019] FIG. 11(b) is a cross sectional enlarged detail of the view
in FIG. 11(a).
[0020] FIG. 12(a) is an isometric of a typical film capacitor.
[0021] FIG. 12(b) is a cross section of the film capacitor shown in
FIG. 12(a).
[0022] FIG. 13(a) and FIG. 13(b) are one example of the physical
arrangement of the tuning capacitor shown in FIG. 10.
[0023] FIG. 14 is another example of the physical arrangement of
the tuning capacitor shown in FIG. 9.
[0024] FIG. 15 is another example of the physical arrangement of
the tuning capacitor shown in FIG. 9.
[0025] FIG. 16 is another example of the physical arrangement of
the tuning capacitor shown in FIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Referring to the drawings, wherein like numerals indicate
like elements, there is shown in FIG. 2 an illustration of one
example of power supply 10 of the present invention for use in
induction heating or melting applications. Ac-to-dc rectifier and
filter section 12 includes an ac-to-dc rectifier. A multi-phase
rectifier, in this non-limiting example of the invention, a
three-phase diode bridge rectifier 14 is used to convert
three-phase (A, B, C) ac utility line power into dc power. Optional
current limiting reactor L.sub.8 smoothes out the ripple from the
output dc current of the rectifier. Section 16 of the power supply
diagrammatically illustrates coil tuning capacitor C.sub.1, which
can be a single capacitor or a bank of interconnected capacitors
that form a capacitive element.
[0027] In FIG. 2, the dc output of the rectifier is supplied to
input terminals 1 and 2 of a full-bridge inverter in inverter
section 20. The inverter consists of solid state switches S.sub.1,
S.sub.2, S.sub.3 and S.sub.4 and associated antiparallel diodes
D.sub.1, D.sub.2, D.sub.3 and D.sub.4, respectively. Alternating
turn-on/turn-off cycles of switch pairs S.sub.1/S.sub.3 and
S.sub.2/S.sub.4 produce a synthesized ac inverter output at
terminals 3 and 4. A preferred, but not limiting, choice of
component for the solid state switch is an isolated gate bipolar
transistor (IGBT), which exhibits the desirable characteristics of
power bipolar transistors and power MOS-FETs at high operating
voltages and currents. In one example of the invention, the
inverter employs a phase-shifting scheme (pulse width control)
relative to the turn-on/turn-off cycles of the two switch pairs
whereby variable overlapping on-times for the two switch pairs is
used to vary the effective RMS output voltage of the inverter.
[0028] Induction load coil L.sub.9 represents the power coil used
in the induction heating or melting apparatus. The capacitance of
capacitor C.sub.1 is selected to form a resonant circuit with the
impedance of load coil L.sub.9 at the operating frequency of the
inverter, which is the switching rate of the switch pairs used in
the inverter. Consequently, a tuning capacitor is not required at
the output of the inverter. Selection of available circuit
components may not allow operation exactly at resonance, but as
close to resonance as is achievable with available components. The
ac current flowing through induction load coil L.sub.9 from the
output of the inverter magnetically couples with an electrically
conductive material, which may be, for example, a conductive metal
or a susceptor.
[0029] FIG. 3 through FIG. 5 illustrate the performance
characteristics for power supply 10 of the present invention as
shown in FIG. 2 with input utility line power (A, B, C) of 480
volts line-to-line, 60 Hertz, and inverter 20 operating at an
output frequency of 60 Hz. For this particular non-limiting
example: L.sub.8 is selected as 5,000 .mu.H (for an impedance of
3.77 ohms at the rectifier ripple output frequency of 120 Hz);
C.sub.1 is selected as 5,000 .mu.F (for an impedance of 0.27 ohms
at the rectifier ripple output frequency of 120 Hz); and L.sub.9 is
selected as 1,000 .mu.H (for an impedance of 0.38 ohms at the
inverter output frequency of 60 Hz). Not shown in FIG. 2, but used
in this sample analysis is a resistance of 0.16 ohms for induction
load coil L.sub.9. Operating the C.sub.1/L.sub.9 circuit at
resonance for the output frequency of inverter 20 results in a
substantially sinusoidal inverter output voltage, V.sub.out and
output current, I.sub.out (at terminals 3 and 4), as graphically
illustrated in FIG. 3. FIG. 4 graphically illustrates that the
voltage across capacitor C.sub.1, namely V.sub.C1, is driven to its
limiting lower value of zero volts as a result of capacitor C.sub.1
being in resonance with coil L.sub.9 at the ripple frequency of 120
Hz. V.sub.C1 is the applied voltage to the input of inverter 20 (at
terminals 1 and 2). FIG. 4 also illustrates the ripple current,
I.sub.L8, through reactor L.sub.8. The impedance of reactor L.sub.8
is generally selected to be much greater than the impedance of
C.sub.1 to block feedback of harmonics from the inverter circuit to
the rectifier's power source. FIG. 5 graphically illustrates the
voltage, V.sub.5, across one of the solid state switches in
inverter 20, and the current, I.sub.S, through one of the switches
at maximum power output when there is zero overlap angle between
V.sub.s and I.sub.s. Switching device turn-off at zero volts for
V.sub.s when dc ripple has reached zero (e.g., at 240.0
milliseconds (ms) in FIG. 4 and FIG. 5), will minimize switching
loses. Additionally, since switching commutation occurs at zero
voltage in this example, any spikes due to stray circuit inductance
will be significantly less than in a conventional inverter having
low ac ripple current in the dc link voltage. This specific example
is provided to illustrate the practice of the invention, which is
not limited to the specific elements and values used in this
example.
[0030] FIG. 6 illustrates a second example of the present
invention. In this example, the load coil consists of an active
coil L.sub.1 and at least one passive coil L.sub.2. Coils L.sub.1
and L.sub.2 may be wound in one of various configurations, such as
sequentially or overlapped, to accomplish mutual magnetic coupling
of the coils as further described below. Coil L.sub.1 is connected
to the output of inverter 20. Coil L.sub.2 is connected in parallel
with resonant tuning capacitor C.sub.2 to form a parallel tank
resonant circuit. Coil L.sub.2 is not physically connected to coil
L.sub.1. The parallel tank resonant circuit is energized by
magnetically coupling coil L.sub.2 with the magnetic field
generated in coil L.sub.1 when current supplied from the output of
inverter 20 flows through coil L.sub.1.
[0031] The benefit of separate active and passive coils can be
further appreciated by the vector diagram shown in FIG. 7. In the
figure, with respect to the active coil circuit, vector OV
represents current I.sub.1 in active coil L.sub.1 as illustrated
FIG. 6. Vector OA represents the resistive component of the active
coil's voltage, I.sub.1R.sub.1 (R.sub.1 not shown in the figures).
Vector AB represents the inductive component of the active coil's
voltage, .omega.L.sub.1I.sub.1 (where co equals the product of
2.pi. and f, the operating frequency of the power supply). Vector
BC represents the voltage, .omega.MI.sub.2, induced by the passive
coil L.sub.2 onto active coil L.sub.1. The half-wave ripple voltage
V.sub.C1 across capacitor C.sub.1 and the switching function of the
two switch pairs S.sub.1/S.sub.3 and S.sub.2/S.sub.4 produce the
effect of a pseudo capacitor C.sub.1' connected in series with
L.sub.1 that would result in a sinusoidal voltage at terminals 5
and 6 in FIG. 6. Vector CD represents the voltage,
I.sub.1/.omega.C.sub.1', that would appear across this pseudo
series capacitor C.sub.1'. Vector OD represents the output voltage,
V.sub.inv, of the inverter (terminals 3 and 4 in FIG. 6).
[0032] With respect to the passive coil circuit, vector OW
represents current I.sub.2 in passive coil L.sub.2 that is induced
by the magnetic field produced by current I.sub.1. Vector OF
represents the resistive component of the passive coil's voltage,
I.sub.2R.sub.2 (R.sub.2 not shown in the figures). Vector FE
represents the inductive component of the active coil's voltage,
.omega.L.sub.2I.sub.2. Vector EG represents the voltage,
.omega.MI.sub.1, induced by the active coil L.sub.1 onto passive
coil L.sub.2. Vector GO represents the voltage,
I.sub.2/.omega.C.sub.2, on capacitor C.sub.2, which is connected
across passive coil L.sub.2.
[0033] The active coil circuit is driven by the voltage source,
V.sub.inv, which is the output of inverter 20, while the passive
coil loop is not connected to an active energy source. Since the
active and passive coils are mutually coupled, vector BC is added
to vector OB, V.sub.LOAD, which represents the voltage across an
active induction load coil in the absence of a passive capacitive
load coil circuit, to result in vector OC, V.sub.LOAD, which is the
voltage across an active load coil with a passive capacitive load
coil circuit of the present invention. The resultant load voltage,
V.sub.LOAD, has a smaller lagging power factor angle, .phi.
(counterclockwise angle between the x-axis and vector OC), than the
conventional load coil as represented by vector OB. As illustrated
in FIG. 7, there is a power factor angle improvement of
.DELTA..phi..
[0034] In the present invention, the inductive impedance in the
passive coil is substantially compensated for by the capacitive
impedance (i.e., (.omega.L.sub.2.apprxeq.1/.omega.C.sub.2). The
uncompensated resistive component, R.sub.2, in the passive coil
circuit is reflected into the active coil circuit by the mutual
inductance between the two circuits, and the effective active coil
circuit's resistance is increased, thus improving the power factor
angle, or efficiency of the coil system.
[0035] Further the power factor angle, .PSI., for the output of the
inverter improves by .DELTA..PSI. as illustrated by the angle
between vector OJ, V'.sub.inv (resultant vector of resistive
component vector OA and capacitive component vector AJ in the
absence of a passive load coil circuit) and vector OD, V.sub.inv
(resultant vector of resistive component vector OH and capacitive
component vector HD with a passive load coil circuit of the present
invention).
[0036] In other examples of the invention multiple active and/or
passive coil circuits may be used to achieve a desired multiple
coil arrangement for a particular application.
[0037] FIG. 8 illustrates another example of the power supply of
the present invention. In this example autotransformer 80 is
connected to the ac output of the inverter. The autotransformer has
a first output terminal and a plurality (at least two) of second
output terminals typically represented by autotransformer taps 100,
110 and 120 in FIG. 8. The first terminal of induction load coil
L.sub.9 is connected to the autotransformer's first output
terminal. The second terminal of the induction load coil is
alternatively connected to one of the plurality of the
autotransformer's second output terminals. The circuit impedance of
the autotransformer changes with the connected tap, which changes
the load circuit impedance so that the power supply in FIG. 8 can
selectively operate at approximate resonance at different output
frequencies from the power supply. This is of advantage, for
example, when an electrically conductive material is being
inductively heated. As known in the art inductively heating at
different frequencies will change the depth of induced heat
penetration of the material. When different depths of heating are
required the tap on the autotransformer can be changed to achieve
this result with the power supply operating at approximate resonant
frequency.
[0038] FIG. 9 illustrates one example of the physical arrangement
for coil tuning capacitor C.sub.1 and inverter elements, namely
solid state switches S.sub.1, S.sub.2, S.sub.3 and S.sub.4 and
associated antiparallel diodes D.sub.1, D.sub.2, D.sub.3 and
D.sub.4, respectively, for the power supply of the present
invention. This arrangement is particularly favorable for
minimizing stray inductance associated with connections to the coil
tuning capacitor and dc connections to the inverter elements. In
this arrangement, coil tuning capacitor C.sub.1 is contained within
enclosure 22 as further described below. In FIG. 9 one or more
physical terminals 24 represent electrical terminal 60 of capacitor
C, as shown in FIG. 2; similarly one or more physical terminals 26
(best seen in FIG. 11(a)) represent electrical terminal 62 of
capacitor C.sub.1. Electrical insulators 25 may be provided for
electrical isolation between the electrical conductors and
enclosure 22. Each solid state switch and its associated
antiparallel diode may be physically provided as an integrally
packaged switch/diode assembly 28a, 28b, 28c and 28d as shown in
FIG. 9 and FIG. 2. The four switch/diode assemblies are connected
to form a full bridge inverter. First switch/diode assembly 28a and
second switch/diode assembly 28b form a first pair of switch/diode
assemblies that both have a first terminal connected to the
positive connection of coil tuning capacitor C.sub.1; third
switch/diode assembly 28c and fourth switch/diode assembly 28d form
a second pair of switch/diode assemblies that both have a first
terminal connected to the negative connection of coil tuning
capacitor C.sub.1. The first terminals of the first and second
pairs of switch/diode assemblies form the dc input to the inverter.
The second terminals of the first and fourth switch/diode
assemblies are connected to a first ac output (AC1) of the
inverter; the second terminals of the second and third switch/diode
assemblies are connected to a second ac output (AC2) of the
inverter. In FIG. 9 physical electrical conductor 30, represented
by circuit terminal 1 in FIG. 2, connects positive capacitor
physical terminals 24 (electrical terminal 60) to the terminals of
switch/diode assemblies 28a and 28b that correspond to electrical
terminals 1 in FIG. 2. Similarly physical electrical conductor 34,
represented by circuit terminal 2 in FIG. 2, and connects negative
capacitor physical terminals 26 (electrical terminal 62) to the
terminals of switch/diode assemblies 28c and 28d that correspond to
electrical terminals 2 in FIG. 2. Physical electrical conductor 36
(via intermediate electrical conductors 36a and 36b joined together
at electrically conductive connection 36c as shown in FIG. 11(a) in
this non-limiting example of the invention) is represented by
circuit terminal 3 in FIG. 2, and connects terminals of
switch/diode assemblies 28a and 28d (corresponding to first ac
electrical terminal 3 in FIG. 2) to a first terminal of induction
load coil L.sub.9 (not shown in FIG. 10). Similarly physical
electrical conductor 38 (via intermediate electrical conductors 38a
and 38b joined together at a suitable electrically conductive
connection not shown in the figures, in this non-limiting example
of the inverter) is represented by circuit terminal 4 in FIG. 2,
and connects terminals of switch/diode assemblies 28b and 28c
(corresponding to second ac electrical terminal 4 in FIG. 2) to a
second terminal of induction load coil L.sub.9 (not shown in FIG.
10). It is one object of the present invention to keep the
inductance in the physical connections between the tuning capacitor
and dc input to the inverter as low as possible. Therefore,
conductors 30 and 34 are preferably formed from a thin sheet
material such as copper and sandwiched together with a thin layer
of high dielectric strength material 33 (such as a MYLAR based
dielectric) between them. Minimal thickness of the conductors and
insulation keeps stray inductance to a minimum. It is also
preferable to keep all dimensions of conductors 30 and 34 to the
minimum required to make suitable connections.
[0039] Similarly it is desirable to maintain a low inductance
circuit for the coil tuning capacitor C.sub.1. In one non-limiting
arrangement of the invention, coil tuning capacitor C.sub.1
comprises one or more wound film capacitors 60 shown in a typical
arrangement in FIG. 12(a) and in partial cross section in FIG.
12(b). First capacitor conductor 61 is separated from adjacent
second capacitor conductor 63 by dielectric layers 62 and 64. First
capacitor conductor 61 extends to the top of the rolled capacitor,
while second capacitor conductor 63 extends to the bottom of the
rolled capacitor. A first electrical conductor in contact with the
top of the rolled capacitor will form the first terminal of the
capacitor and a second electrical conductor in contact with the
bottom of the rolled capacitor will form the second electrical
conductor.
[0040] In the arrangement shown in FIG. 13(a) and FIG. 13(b),
capacitors 60a and 60b are arranged on opposing sides of first and
second capacitor connecting electrical conductors 66 and 68, which
are electrically separated by a dielectric 67. As with the
conductors between the terminals of the coil tuning capacitor and
the dc input to the inverter, in order to kept the inductance low,
conductors 66 and 68 are preferably formed from a thin sheet
material such as copper and sandwiched together with a thin layer
of high dielectric strength material 67 (such as a MYLAR based
dielectric) between them.
[0041] Capacitors 60a have their second (bottom) capacitor
conductors 63 electrically in contact with first connecting
electrical conductor 66. Capacitors 60b have their first (top)
capacitor conductors 61 in contact with second connecting
electrical conductor 68. Capacitors 60a have their first (top)
capacitor conductors 61 electrically in contact with second
connecting electrical conductor 68 by electrical conductor 70a, and
capacitors 60b have their second (bottom) capacitor conductors 63
electrically in contact with first connecting electrical conductor
66 by electrical conductor 70b. Electrical conductors 70a and 70b
may be in the form of a copper rod passing through the center
(spool) of each capacitor with an extending electrical conducting
element at each end so that the first end of the copper rod makes
contact with a capacitor's conductor that is not in contact with
either connecting electrical conductor 66 or 68, and the second end
makes contact with either connecting electrical conductor 66 or 68.
Electrical insulation 67 is provided around electrical conductors
70a and 70b so that they do not make electrical contact with a
connecting electrical conductor that would short out a capacitor.
The extending electrical conducting element may be in the form of a
copper plate 70c. Connecting electrical conductors 66 and 68 extend
out of enclosure 22 to form first and second capacitor terminals 24
and 26.
[0042] In the alternative arrangement shown in FIG. 14, capacitors
60c have their second (bottom) capacitor conductors 63 electrically
connected to first connecting electrical conductor 66. The first
(top) capacitor conductor 61 of each capacitor 60c is electrically
connected to second connecting electrical conductor 68 via
electrical conductors 70a with suitable extending electrical
conducting elements 70c.
[0043] In the alternative arrangement shown in FIG. 15, first
connecting electrical conductor 66 may be press fitted around one
or more capacitors 60d. In this arrangement first (top) capacitor
conductor 61 makes electrically contact with connecting electrical
conductor 66 and second (bottom) capacitor conductor 63 makes
electrical contact with connecting electrical conductor 68.
[0044] In the alternative arrangement shown in FIG. 16, first and
second connecting electrical conductors 66 and 68 may be press
fitted around one or more capacitors 60e. In this arrangement first
(top) capacitor conductor 61 makes electrically contact with
connecting electrical conductor 66 and second (bottom) capacitor
conductor 63 makes electrical contact with connecting electrical
conductor 68.
[0045] In all alternative arrangements of capacitors, conductors 66
and 68 are preferably formed from a thin sheet material such as
copper and sandwiched together with a thin layer of high dielectric
strength material 67 between them.
[0046] The examples of the invention include reference to specific
electrical components. One skilled in the art may practice the
invention by substituting components that are not necessarily of
the same type but will create the desired conditions or accomplish
the desired results of the invention. For example, single
components may be substituted for multiple components or vice
versa. Further one skilled in the art may practice the invention by
rearranging components to create the desired conditions or
accomplish the desired results of the invention. While the examples
illustrate operation of the invention in full-bridge voltage-fed
power supplies, the invention is applicable to other power supply
topologies with appropriate modifications as understood by one who
is skilled in the art.
[0047] The foregoing examples do not limit the scope of the
disclosed invention. The scope of the disclosed invention is
further set forth in the appended claims.
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